1. Field of the Invention
The invention described and claimed herein relates to the preparation of powdery paraformaldehyde from a concentrated solution of formaldehyde.
2. Description of the Prior Art
Formaldehyde is a large volume chemical with a wide variety of industrial uses. Much formaldehyde is sold and used as a water solution. However, during recent years an increasing amount has been sold in solid form as paraformaldehyde.
Paraformaldehyde is a mixture of polymers of formaldehyde of different molecular weight. Paraformaldehyde is usually made by concentration of a formaldehyde solution and solidification of the concentrated solution by cooling. Paraformaldehyde is available as powder, flakes, or prills, and in this form is generally storage stable, convenient to transport and readily soluble in water or alcohol, in which the paraformaldehyde readily depolymerizes to form solutions of formaldehyde.
It is known that a free-flowing paraformaldehyde product can be produced by spraying an aqueous formaldehyde concentrate containing 80-90 weight percent of formaldehyde downwardly into a spray cooling chamber while passing a current of air having a temperature between 40.degree. and 60.degree. C. upwardly through the cooling chamber. The chamber is so constructed as to maintain a fluidized bed of the sprayed particulate product in the lower portion thereof until dry, nonsticky particulate paraformaldehyde is formed. Such a process is described in Mann et al. U.S. Pat No. 3,316,309 and, while said to be operative on a small scale, has been criticized in a later Mann et al. patent as not satisfactory for large scale technical operations since the small paraformaldehyde spheres which are the primary product are not sufficiently free of tackiness and quickly agglomerate causing the process to come to a standstill because of complete clogging of the nozzles employed, the spray chamber, the drying air passages and particularly the sieve grates through which air is supplied to the fluidized bed maintained in the lower portion of the spray dryer.
An attempt to solve the problems of the aforementioned process is described in Mann et al. U.S. Pat. No. 3,595,926, which discloses and claims a process for preparing paraformaldehyde in which an aqueous formaldehyde concentrate containing 80-90 weight percent of formaldehyde is sprayed downwardly into a spray cooler constructed so as to maintain a fluidized bed of the sprayed particulate product in the lower portion of the cooler. It is a critical aspect of this latter process that the temperature of the cooling gas in the cooling chamber be quite cool, preferably between -40.degree. C. and +30.degree. C. and that the paraformaldehyde spheres remain in the fluidized bed of the cooling tower until they have been cooled to a temperature below 40.degree. C., preferably 20.degree.-30.degree. C. in order to avoid stickiness. The cooled pellets are then dried in a contact dryer to a final formaldehyde content of 92-97%. Needless to say, a process which requires considerable cooling of the product may in many climates necessitate the purchase of special refrigeration equipment thereby raising the overall cost and complexity of the process.
In U.S. Pat. No. 3,772,392 there is shown a process for preparation of paraformaldehyde by spraying concentrated formaldehyde solution into a spray cooler and subsequently curing the product. A polymerization catalyst or a regulator is required to regulate curing of the product which occurs on a band conveyor prior to any drying of the paraformaldehyde. The cured product is transferred to a hot air dryer or a fluidized bed for subsequent drying and concentration.
It would be highly desirable that a spray cooling process for producing paraformaldehyde be capable of functioning without the use of polymerization catalyst or regulator. It would be further desirable that a spray cooling process for producing paraformaldehyde be capable of functioning without the necessity of maintaining a fluidized bed in the bottom of the spray cooling tower. It is still further desirable that a spray cooling process for producing paraformaldehyde permit the removal of a powdery product from the spray cooling tower in a somewhat sticky or agglomerated condition at temperatures above 40.degree. C.